Timbuktu

Take a tower in a town.
Face it.
Compare the apparent widths of its base and of its top.
Notice the latter is smaller.
Conclude the tower is apparently trapezoidal.

In case you think it's a classical perspective effect (i.e. all lines seem to converge to a fixed point), repeat the experience, but now with a very broad building (a palace if you can ; or if you liked it when it was vertical, imagine yourself at the top of a tower adjacent to the 8:46 Twin Tower...), and compare the apparent height of its centre to those of its lateral extremities.
Notice the latter are smaller.
Conclude the palace is apparently an almond.


I remember reading in an art theory book (possibly by Gombrich) that this effect is merely an illusion because you actually have to turn your head to compare the different part of the building, so that in the end you're comparing quantities that shouldn't be... I was never able to make sense of this argument : even with my eyes fixed on the centre, my peripheral vision still allows me to perceive this almond effect.

Now most of the people would agree that the edges of the "real" tower -- not the apparent one -- are of course parallel (they would prove it by taking a ruler and measuring the width of the tower at different places). So this tells us something about the meaning of the word "real" : what is considered "real" is not what we perceive but rather an artificially reconstructed idealisation of the perceived objects, that is abstracted from an infinite series of representations acquired from different points of view.

In other words, to "see" a physical object, it is necessary to look at it from all possible sides, travelling across an entire virtual globe around it -- or rotating it in all possible ways in our hand. But in fact we quickly get so good at it during our formation years that just a few glimpses at an object (provided it is not too extraordinary) and we are able to construct a very efficient mental image of its "real" shape.

What about its colour ? Perhaps you would say its colour is what you see when it is lit with pure white light, but you have to take into account the particularities of your visual organs (imagine you're daltonian). The "true" colour of an object is naturally taken to be the colour that most of us would agree upon, i.e. it is defined intersubjectively. [Aristotelian philosophers referred to the colour as an example of "secondary quality" and opposed it to "primary qualities" such as the shape. Throughout history the number of primary qualities kept on decreasing, until it reached absolute zero with Hume (not quite in fact maybe...). My belief is that this distinction is not pertinent, that every quality is secondary.]

Go further : what does the concept of "liberty" "really" mean ? [yes, I know, I am not fully happy with my use of inverted commas myself...] It is also clear that the definition of any concept will require some sort of concertation between the individuals of the group within which the concept is in use. So let me formalize that in the following

Lemma 1.1 The "reality" of an object (either physical or conceptual) is determined by intersubjective agreement.


Keep that in mind, switch it off, and tell me what you see in the picture below. Nothing ? Sure ?


Still not ? What about now ?


Yes, no ? HA ! NOW you see the Dalmatian don't ya ? [Shit I start talking like Palin now ; )]
I saw this picture in an online lecture about cognitive psychology and the lecturer asked the lady that said "HA" the loudest what happened on the screen at that moment. She replied "On the screen nothing, in my brain something happened," which the lecturer, very pleased, repeated : "On the screen nothing, but in the brain -- I couldn't have said any better." At the time I found it rather obvious (selbstverständlich as the German say), but this morning when I woke up I realized it wasn't so. (I should have been alarmed by the fact that the expert's analysis coincided with the old lady's plain common sense, whereas one would expect that profound science would reach the extreme boundaries of common sense and beyond...)

It is NOT obvious that the "real" picture on the screen did not change before and after the realisation of its representational content. Just use Lemma 1.1 : once the entire audience has identified the representation of the Dalmatian in the picture (and you too, and me), it CANNOT be denied its share of "reality".

(Without elaborating any further, I suspect that this kind of considerations should reach their full flavour in the context of the foundations of quantum mechanics...)

(Rigid philosophers like Bertrand Russell would use arguments such as "The table I write on, I can touch it, so it exists, and it exists even if I close my eyes" etc. My impression is that he is under the same spell than the old lady, were she asked to "un-see" the Dalmatian once she's seen it. Very difficult to do, I'll have you know ! The only way I can get myself to do it is by focusing on a small fraction of the picture. That again seems to indicate that "de-realisation" effects are to be expected in the microscopic realm.)